The invention relates to a sheet metal blank.
In particular the invention relates to a casing made from a sheet metal blank.
Casings for gas turbine engines, for example compressor outer casings, combustor outer casings and turbine outer casings, are frequently made from castings or forgings which are then machined to the correct dimensions. Alternatively a casing may be fabricated from sheet metal, with end flanges and bosses being welded into place which are machined to final dimensions to interface with other components, to provide access to the engine when assembled, for the passage of pipes and services from the exterior of the interior of the engine (eg fuel pipes and cables) or to accept external fittings. Since the casings are typically very thin (perhaps only 1 to 5 mm thick) it is common for slight misalignments in the welding of the casing to result in damage to the casing. Modern engines frequently require a great many bosses on engine casings, and the consequential distortion due to welding at multiple locations can significantly affect the profile of the finished casing. Hence there is a high possibility of damage to the casing during the manufacturing process.
Hence a casing having the required number of features, strength and geometry which can be produced by a method resulting in less component rejections is highly desirable.
The production of the base casting, forging or sheet metal shell to which bosses are welded to is also a problematic process and results in many rejections, slowing the manufacturing process and increasing the over all cost of each component which is successfully produced.
Hence it is desirable that the base structure that each casing is formed from is as easily and cheaply produced as possible.